According to the principles of special relativity, time dilation occurs when an object moves at speeds approaching the speed of light. As an object accelerates and approaches the speed of light, time for that object appears to slow down relative to an observer who is at rest.
In the scenario you described, if you were to travel in a spaceship at the speed of light for 5 years (according to your own measurements onboard the spaceship), a phenomenon known as time dilation would occur. However, it's important to note that it is not possible for massive objects with mass to reach or exceed the speed of light. The theory of relativity specifically prohibits it.
Nonetheless, for the sake of this hypothetical scenario, let's assume that you were somehow able to travel at the speed of light. From your perspective on the spaceship, only 5 years would have passed. However, for your friends who remained on Earth, time would have continued to progress at its normal rate.
Due to time dilation, the time experienced by your friends on Earth would appear to be passing much more quickly than the time you experienced in the spaceship. As a result, when you return to Earth after 5 years, according to your own aging process, you would be 20 years old. However, your friends on Earth, who experienced 65 years during that time, would indeed be much older than you.
It's important to note that this scenario is purely theoretical, as traveling at the speed of light is currently considered impossible for objects with mass. The example serves to illustrate the consequences of time dilation as predicted by the theory of relativity.